News - November 29, 2010

Bacteria guide malaria mosquitoes

The composition of our skin bacteria determines whether we are attractive to malaria mosquito. This insight should make it possible to develop an effective odour trap for mosquitoes.

The various species of bacteria on our skins turn sweat into specific odours which malaria mosquitoes recognise with great precision. Staphylococci are attractive to the malaria mosquito, whereas pseudomonas bacteria are less so. The more different types of skin bacteria you harbour, the more attractive you are to the mosquito. This discovery was made by PhD researcher Niels Verhulst.

The characteristic odour of human sweat does not consist of sweat alone, explains Verhulst. People only begin to stink once the skin bacteria have got to work on the sweat - that is why you do not smell sweat straightaway.
Sweat odour
Verhulst tested how mosquitoes reacted to five skin bacteria, both in Wageningen and in Kenya. He then tested the sweat odour of 48 volunteers on the Anopheles gambaie mosquito. One skin bacterium - the pseudomonas - did not attract a single mosquito. The other four all generate odours that attract mosquitoes, Verhulst discovered.
In order to find our which odours made the sweat so attractive, Verhulst had odour analyses done in Germany. About fifteen odours were identified. It is hoped that further research in Kenya will reveal for each bacterium which odours attract the mosquitoes.
The Gates Foundation funds Verhulst's research. The aim is to develop an odour trap for mosquitoes. A handful of odours that lure mosquitoes into a trap have already been identified, but they are not successful enough. The mosquitoes still prefer human sweat to these isolated odours. 'We now have a few odours that work well', says Verhulst. 'We keep on adding odours to these top candidates, and then the mosquitoes let us know which odour they find most attractive. Like this we hope to get a more and more complex and effective blend.'
A human being (plus skin bacteria) produces about 300 different odours. Getting the right combination of these odours for an odour trap is a tricky job. One bacterium produces about fifteen odours. So it seems easier to create an odour blend via the five skin bacteria.
Verhulst has also researched whether our attractiveness for mosquitoes is genetically based. Previous research had shown that genes determine our odour profile. If women smell the T shirts of ten men, they pick out the one belonging to the man with the gene profile the most different to their own. Could the same process be at work among mosquitoes? Verhulst has evidence that one particular gene corresponds strongly with our attractiveness to mosquitoes, but the correlation fell just short of statistical significance.
Verhulst is due to get his PhD on 9 December from Entomology professors Willem Takken and Marcel Dicke.